Quite a useful bit of kit in tidal waters because it measures speed through the water rather than over the ground.

Example of use: if you position yourself beside a stationary object such as a buoy you can measure actual tidal flow (direction and speed) which you can then use to plot a direct course to your destination.

This might not seem anything to get terribly excited about but if you do not have a working GPS and find yourself needing to get from point A to point B in restricted visibility (e.g. fog or night) it could be a potential lifesaver.